1. Field of the Invention
The present invention generally relates to wiring boards, and more particularly, to a wiring board having a solid surface on which discrete parts and/or a thin-film multilayer structure is provided and to a wiring board having a thick-film multilayer wiring layer structure formed therein.
2. Description of the Related Art
FIG. 1 shows an aluminum nitride (AlN) board related to the present invention (based on the inventor's knowledge). An AlN board 10 has inner conductive wiring layers 11 and 18 formed in a stacked formation. The inner conductive wiring layers 11 and 18 are made of an electrically conductive material such as tungsten (W) or molybdenum (Mo). The stacked formation of the wiring layers can be formed by a thick-film printing technique and a burn-in technique. Generally, a metallic layer 12, which may have a Cu/Ni/Au structure, is formed on the back surface of the Al board 10 taking into account the electrical performance. A cover pad 13 is provided on the front surface of the AlN board 10 (in other words, a circuit pattern formation surface). The cover pad 13 covers the inner conductive wiring layer 11 extending up to the front surface from an inner portion of the board 10. Generally, the cover pad 13 has a Ti/Cu/Ni/Au structure. The cover pad 13 is electrically connected to the inner conductive wiring layer 11, and is used as a pad provided on a peripheral surface area of the board 10 and is used to input or output a signal. There is another cover pad 14, which is not connected to any inner conductive layer. Generally, the cover pad 14 has the same structure as the cover pad 13, and is used to fix a casing on the board 10. Electronic parts (elements) such as LSI chips are mounted on the board 10 and then a cover is provided to hermetically seal the board 10 and the parts.
A multilayer wiring structure 15 is provided on the front surface of the AlN board 10. The multilayer wiring structure 15 has a stacked unit made of copper and polyimide. Copper wiring layers 16 are stacked between polyimide layers 17. Via holes are formed in the polyimide layers 17, and the upper wiring layers 16 are electrically in contact with the lower wiring layers 16 or an inner conductive wiring layer 18. For example, the copper wiring layers 16 are 25 .mu.m wide and 7 .mu.m thick, and the polyimide layers 17 are 25 .mu.m thick. A soldering resist layer 19 made of polyimide is provided on the top of the uppermost copper wiring layer 16. A die pad metallic layer 20 is formed on the front surface of the AlN board 10.
FIGS. 2A, 2B and 2C are diagrams showing the structure of the AlN board 10 and its production method. As shown in FIG. 2A, a stacked-layer structure is formed. A plurality of insulating layers 10-3 made of AlN are stacked and dummy layers 10-1 and 10-2 formed of AlN are provided as shown in FIG. 2A. Inner conductive wiring layers 10-4 are formed in the dummy layers 10-2 and the insulating layers 10-3. The inner conductive wiring layers 10-4 correspond to the aforementioned inner conductive wiring layers 11 and 18 shown in FIG. 1. The inner conductive wiring layers 10-4 extend in the vertical and horizontal directions. The above stacked structure shown in FIG. 2A is baked. Then, the dummy layers 10-1 and 10-2 are polished. The inner conductive layers 10-4 are formed by the thick-film printing technique, and therefore the surface thereof has a roughness 10-7 equal to a few microns as shown in FIG. 2A. A conductive thin film cannot be formed on such as rough surface. Thus, the dummy layers 10-1 and 10-2 are polished so that the inner conductive wiring layers 10-4 have flat exposed surfaces, as shown in FIG. 2B. Thereafter, as shown in FIG. 2C, cover pads 10-5 are provided so as to cover the exposed surfaces of the inner conductive wiring layers 10-4.
Generally, AlN material is eroded and melted (etched) by an alkali solution which may be NaOH or KOH. The cover pads 10-5 shown in FIG. 2C are formed by growing a film on the entire surface of the structure by means of sputtering chromium (Cr) and then patterning the film. In the patterning process, a strong alkali solution (pottasium ferricyanide plus pottasium hydroxide) is used as an etching resist and etching liquid. Hence, as shown in FIG. 2C, in the process of forming the cover pads 10-5, the alkali solution infiltrates into the AlN layers 10-3, gaps 10-6 then being formed around the inner conductive wiring layers 10-5. The gaps 10-6 causes defective contacts between the inner conductive wiring layers 10-5 and the AlN layers 10-3 as well as defective via contacts. The above problem will occur even when an alkali solution is used to etch a metal other than Cr, such as Cu or Ti.
It may be possible to use a metal etched by a solution other than an alkali solution to form the pattern by a process not using any alkali solution. However, there will be large restrictions on the process of forming the pattern, and a good-quality multilayered board (wiring structure) cannot be realized.